1. Field of the Invention
This invention relates generally to wavelength dispersion in optical systems.
2. Background Art
Molecular fluorine gas lasers are employed to produce powerful deep ultraviolet laser light sources. These light sources are used in a variety of applications, such as atomic and molecular research, optic and mechanical component development, and photolithography.
Molecular fluorine gas lasers typically emit ultraviolet light in the 156-158 nanometer (nm) spectral region (hereinafter referred to as the xe2x80x9cultravioletxe2x80x9d laser emission). The molecular fluorine gas laser also emits light in the spectral region between 600 nm and 800 nm, in the visible to near infrared spectral region. This emission band is hereinafter referred to as the xe2x80x9credxe2x80x9d emission.
The presence of this red emission is often undesirable for applications which employ the ultraviolet emission, such as photolithography. Various techniques are available to reduce or eliminate the red emissions. These techniques include modifications to the laser""s gas composition, wavelength dispersive technology, and wavelength selective optical coatings. However, these techniques can reduce the performance of the laser.
For example, one technique modifies the gas composition of the laser to reduce or eliminate the red light emission, but it also reduces overall laser performance, especially at high (multi-kilohertz) repetition rates desirable for laser photolithography (e.g., as in pulsed or switching laser implementations).
In one specific example, neon gas is substituted for helium gas in a laser gas mix to reduce or eliminate the red light. The disadvantages of neon are that it is much more expensive than helium and that it can reduce performance at high repetition rates.
Furthermore, the red light emission from the molecular fluorine gas laser has significant superradiant character that it is not effectively removed by current techniques. For example, it is well-known that the introduction of laser spectral line selection technology at the high reflector end of the laser resonator will have a wavelength dispersive effect. Unfortunately, molecular fluorine gas lasers using such dispersive laser line selection technology can still emit 2% or more of the output energy in the red emission component.
Additionally, selective coatings on optical surfaces that are either internal or external to the laser resonator can reduce the red emission component, but complicate the optical delivery system. For instance, wavelength-selective optical coatings can be used to remove the red emission, but they typically degrade the reliability, durability and/or performance of the laser.
Therefore, in view of the above, what is needed is a method and apparatus for optimizing the output beam characteristics of a laser. Such a method and apparatus needs to remove the red emissions from the output beam without compromising laser or optical system performance. Further, what is needed is a method and apparatus that can remove the red emission component without reducing the effective output energy of the laser.
The present invention uses refractive optical elements, such as lenses, to reduce or eliminate unwanted wavelength components from the emission of a laser source while minimizing any optical losses introduced by the method and apparatus. A method and apparatus reduce losses over existing techniques and improve overall laser performance. System performance is improved by reducing or eliminating the red emission in a broad range of applications. In one embodiment, the present invention eliminates the red light emission from an output beam of a laser. In another embodiment, the present invention eliminates the red light emission from a beam monitoring and/or beam regulation component.
The method modifies the parallelism of optical components to disperse wavelength components from the emissions of a laser.
One advantage of the invention is that it enhances the performance of a laser beamsplitter by dispersively separating red light emissions from ultraviolet light emissions in the output laser beam.
Another advantage of the invention is that it improves design control of the fraction of light extracted for output monitoring or pulse energy regulation. Yet another advantage is that the invention can separate the red and ultraviolet components for the monitoring and regulation functions.
Further embodiments, features, and advantages of the invention, as well as the structure and operation of the various embodiments of the invention, are described in detail below with reference to the accompanying drawings.